Kim Jong-il's death has shifted the world's focus to his likely successor, his youngest son Kim Jong-un.

If the planned succession proceeds without a hitch, the little-known, Swiss-educated Kim will be the leader of a nuclear state wracked by instability.

Reports from North Korea said the country's 24 million people had already begun to rally round Kim Jong-un, who has been the communist state's leader-in-waiting for more than a year.

The North's official Korean Central News Agency said the country, people and military "must faithfully revere respectable comrade Kim Jong-un. At the leadership of comrade Kim Jong-un, we have to change sadness to strength and courage and overcome today's difficulties."

Little is known about Kim Jong-il's third son, who is believed to be in his late twenties; reports from Pyongyang suggest his ailing father had spent the past year grooming Kim Jong-un, whom the state media are now calling "the great successor" for leadership of the world's most isolated nation by taking him on "field guidance" trips around the country.

The Swiss-educated Kim Jong-un is believed to have accompanied his father on a trip to China in May this year, apparently in an attempt to win support for his succession from Beijing, North Korea's only remaining ally and a major donor of aid.

He is also reported to have travelled to China with his father in August 2010, when Kim senior - known as the "Dear Leader" - is thought to have met Chinese president Hu Jintao and appealed for diplomatic and financial support for his son's succession.

North Korean state media have mentioned Jong-un in glowing terms, amid fears that his succession could meet with resistance from the upper ranks of the powerful Korean People's Army.

It is possible that Kim Jong-un, who lacks the years of political experience his father enjoyed before becoming leader in 1994, will initially act as the figurehead of a regime run by influential members of the ruling party, according to Yang Moo-jin, a professor at the University of North Korean studies in Seoul.

"Chances that the North Korean military is attempting a coup are very low, because North Korea has called itself a nation sharing a common destiny, Kim Jong-un," he said.

"I think the collective leadership of the party, government and military will go on for a while, because Kim Jong-un is still young."

Yang said South Korea, which has put its military on high alert, would probably attempt to quickly establish a working relationship with the new leadership.

"Now, South Korea urgently needs to think of who in North Korea it has to deal with," he said. "South Korea doesn't want any instability in North Korea, so will probably work to expand its co-operation efforts."

South Korean media have reported that Jang Song-thaek, Kim Jong-il's brother-in-law and a trusted member of his inner circle, had been acting as a mentor to Kim Jong-un in anticipation of a more steady transition of power.

But as it is, Kim senior, who suffered a stroke in 2008 and was reportedly ill with heart disease, died a year short of the centennial of the birth of his father, an event he was to mark by proclaiming his country a successful, modern state and a bona fide nuclear power.

If the handover of power in North Korea, the world's only communistdynasty, proceeds as envisaged by Kim Jong-il, his son could be about to complete a rapid rise to prominence following his public debut in September last year.

Then, he was awarded the rank of four-star general and made vice-chairman of the ruling Workers' party of Korea central military commission, a move analysts say was designed to place him at the centre of the country's power structure.

If he does become the third member of his family to lead North Korea - his grandfather, Kim Il-sung, founded the state in 1948 - Kim Jong-un will face immediate challenges.

The regime had recently been seeking tens of thousands of tonnes of aid amid reports of serious food shortages this winter; a famine in the 1990s killed an estimated 1 million of North Korea's 23 million people.

He will also face renewed pressure to end North Korea's nuclear weapons programme. Two nuclear tests, in 2006 and 2009, sparked strict international sanctions that have placed the leadership under serious financial strain.

Multiparty talks involving the two Koreas, China, the US, Russia and Japan have not been held since Pyongyang walked out of the process in April 2009.

Last year it emerged that North Korea had built a uranium enrichment plant that could give it the ability to drastically strengthen its nuclear capability.

It is known to have enough fissile material from its plutonium-basedprogramme to make between six and 12 nuclear bombs, but has not proved it has a working nuclear weapon.

Kim Jong-il passed over his first and second sons as potential successors, but there are suggestions that he was never convinced that Kim Jong-un would make a successful leader.

Early this year, his eldest son, Kim Jong-nam, told a Japanese newspaper that his father opposed continuing the family dynasty, but had named his youngest brother heir to maintain stability.

In an interview with the Tokyo Shimbun, Kim Jong-nam said that hereditary succession "does not fit with socialism, and my father was against it as well".

Kim Jong-nam fell out of favour with his father in 2001 after he was caught trying to enter Japan on a fake passport, saying he wanted to visit Tokyo Disneyland.

"My understanding is that [succession] was to stabilise the internal system. An unstable North Korea leads to instability in the region," he said in an interview in a southern Chinese city this month.

In a country that, despite its communist doctrine, retains a Confucian respect for seniority, Kim Jong-un could have expected to give way to his older siblings, but reportedly emerged as his father's favourite after impressing him with his single-mindedness and leadership qualities.

The leader-in-waiting, who shares his father's chubby frame, was educated at a Swiss boarding school, where he is said to have excelled at skiing and basketball, and learned to speak English, German and French.

In a 2003 book, Kenji Fujimoto, a Japanese chef who worked as Kim Jong-il's personal cook before fleeing back to Japan, described Kim Jong-un as "a chip off the old block, a spitting image of his father in terms of face, body shape and personality".

Kim Jong-un's presence at North Korea's biggest-ever military parade, in Pyongyang in October last year, was taken by some to mean that he had been accepted by the leaders of the country's 1.2-million strong army.

Kim Yong-hyun, an expert on North Korea at Seoul's Fdongguk University, said at the time." The parade served as a sign that the military has loyalty to the successor."

It remains to be seen how the North Korean establishment and the country's impoverished people react to his possible accession. In leaked US embassy cables published by WikiLeaks last November, South Korean analysts warned the assistant secretary of state, Kurt Campbell, of possible instability arising from a botched succession.

"Of the five experts, one thought the younger Kim might succeed and one argued his lack of leadership experience made it unlikely he would win the support of the ruling elites," the cables said.

They continued: "They agreed that Kim Jong-il's brother-in-law Jang Song-taek would prove a strong rival for the younger Kim and would probably be tempted to challenge him. Kim Jong-il had used draconian controls and international aid to discourage coups after having foiled three such attempts in the late 90s."

The cables referred to doubts about Kim Jong-un's ability to fend off challenges to his leadership in the event of his father's death. The experts noted that the younger Kim "had very little experience and might not get much direct guidance before Kim Jong-il dies".

His father's sudden death, reportedly of heart failure while travelling by train on Saturday morning, has lent even more weight to doubts about his ability to quickly fill his father's shoes.